Question

State the number of molecules present in crystals of washing soda and plaster of Paris. What are these molecules called?

Answer 1

Certain compounds contain water molecules in them. This is very important to maintain the structural integrity of the compounds. These water molecules are present inside the crystal lattice of these compounds.

Complete step by step answer:
Let’s learn about this now. Many salts introduce water molecules into the structure when crystallized from an aqueous solution. This is referred to as 'water of crystallization'. As a definition, we can say, the water of crystallization is the fixed number of water molecules which is present in one formula unit of a compound.
Now coming to the question, the number of water molecules in washing soda is $10$ . This is because the chemical formula of washing soda is $N{a_2}C{O_3}$ $.10{H_2}O$ . This compound is called Sodium Carbonate decahydrate. Hence, it contains ten molecules of water. Whereas, in the plaster of Paris, the chemical formula is $2CaS{O_4}.{H_2}O$ . Here we can see that two molecules of calcium sulfate are sharing one molecule of water. Hence, for one molecule of calcium sulfate, half a molecule of water is required. This means the water of crystallization of the plaster of Paris is $0.5$ .

Additional Information:
The plaster of Paris is produced by heating Gypsum at $373K$ . Plaster of Paris got its name from the history of usage of this material in the houses of people in Paris for plaster purposes in the beginning. Washing soda, which is sodium carbonate helps in softening water and thereby helps in cleaning clothes etc.

Note:
In the case of the plaster of Paris, we have seen that the water of crystallization is half molecules. However, there is no such thing as half a molecule of water. We derive this number from the chemical formula of the substance.